1. Field of the Invention
The present invention relates to a method and a device for optical add/drop multiplexing to be used in extracting or inserting optical signals of a specific wavelength alone to/from optical wavelength division multiplexed signal lights in a plurality of wavelengths.
2. Description of the Background Art
A conventionally known example of an optical add/drop multiplexer (ADM) device has a configuration as shown in FIG. 1, where a fiber grating is used as a wavelength selection filter.
This optical ADM device of FIG. 1 comprises two optical circulators 71 and 75, and a fiber grating 73 provided therebetween, where the optical wavelength division multiplexed signal lights are entered into the first optical circulator 71 at an input port 71a. These signal lights are then circulated by the first optical circulator 71 from the input port 71a to an output port 71b, and entered into the fiber grating 73. The fiber grating 73 reflects only those signal lights in a specific wavelength alone among the entered optical wavelength division multiplexed signal lights, toward the first optical circulator 71. The reflected signal lights are then dropped as DROP signal lights (abbreviated hereafter as DROP lights) from a drop port 71a of the first optical circulator 71.
The signal lights in the other wavelengths which are not reflected by the fiber grating 73 will be transmitted through the fiber grating 73, and entered into the second optical circulator 75 at an input port 75a. These signal lights are then circulated by the second optical circulator 75 from the input port 75a to an output port 75b, and outputted from the output port 75b.
In addition, the second optical circulator 75 has an add port 75c from which ADD signal lights (abbreviated hereafter as ADD lights) in a specific wavelength are to be entered. The ADD lights are then circulated by the second optical circulator 75 from the add port 75c to the input port 75a, and entered into the fiber grating 73. The ADD lights entered into the fiber grating 73 are reflected by the fiber grating 73 and outputted from the output port 75b of the second optical circulator 75 along with the other signal lights which transmitted through the fiber grating 73 as described above.
In this conventional ADM device using the fiber grating, there has been a problem that the DROP lights and the ADD lights to be reflected by the fiber grating are actually not reflected completely, and some DROP lights and ADD lights will be transmitted through the fiber grating to directions opposite to their intended directions.
These transmission components depend on the rejection of the fiber grating. The DROP lights and the ADD lights have the same wavelength so that the leaked DROP light components, that is the leakage of the DROP lights into the ADD lights, interfere with the ADD lights, while the leaded ADD light components, that is the leakage of the ADD lights into the DROP lights, interfere with the DROP lights, and there has been a problem that the beat noises due to these interferences cause the degradation of the transmission characteristic.
In constructing an optical submarine cable system to be used for an optical wavelength division multiplexing communication, for example, it is necessary to account for a case of the worst penalty, and in order to realize a satisfactory level with no significant degradation of the transmission characteristic due to the beat noise between the ADD lights and the leaked DROP light components and the beat noise between the DROP lights and the leaked ADD light components, it is necessary to have about 50 dB of a difference between the signal light and the leaked light as indicated in FIG. 2.
However, currently, no fiber grating that can satisfy 50 dB of the rejection has been manufactured. Even if it is possible to manufacture such a fiber grating in future, it is expected to be quite expensive. According to the current fiber grating manufacturing technology, the rejection is about 35 dB at most, as can be seen from the transmittivity indicated in FIG. 3.
Consequently, the degradation of the transmission characteristic due to the beat noises has been the major problem in the system construction using the optical ADM device.